It’s almost impossible to replace the bottom cover without removing the seat. You have a BigHorn with a power seat and I believe three motors with associated brackets and bars. A bunch of wiring and heating and cooling vent under the seat. I know just installing neoprene seat covers that the driver seat is a royal PITA to attach under there. The OEM cover is held on by hog rings. Small metal circles that are clamped in place by a special tool. The rings are pretty stout and do not bend into place easily. The cover is also stretched over the cushion so it would be difficult to get any sort of leverage with the seat in the truck even if all the power motors and wiring weren’t there. Remove the seat, detach the lower cushion and buy a hog ring tool. If you don’t want to go that route just take the lower cushion and replacement cover to an auto upholstery place, it’s a quick job for them. Taking the seat out will be fairly easy. Taking the seat apart to get the lower cushion off is not so easy but you’d be able to flip the whole seat upside down and make the repair with the whole seat intact out of the truck.